The present disclosure relates to a light emitting device, and more particularly, to a light emitting device that may provide a high color rendering white light similar to a natural light.
A light emitting diode (LED) is a highly efficient and environmental-friendly light source and is taking center stage in various fields. For example, the LED is being used in many fields such as a display device, a car, and general lighting. In particular, a demand for a white-light emitting device implementing a white light is gradually increasing.
A phosphor in the LED functions as a medium that changes energy of an excitation source to energy of a visible light and the efficiency of the phosphor is an important element associated directly with the efficiency of a display product.
There is a light emitting device using a blue LED as one of light emitting devices emitting a white light. The light emitting device using the blue LED uses a blue light as an excitation source to apply a yellow phosphor emitting a yellow light to a LED emitting a blue light, so the blue light emitted from the blue LED is mixed with the yellow light emitted from the yellow phosphor to implement a white color.
The light emitting device emitting the white light uses a way of applying a YAG:Ce phosphor showing a yellow color to the blue LED to obtain the white light, as a method of applying a phosphor to the blue LED to use a blue light emitted from the blue LED and a second light source emitted from the phosphor.
However, there are limitations in that the method involves quantum deficits resulting from using the second light and efficiency decrease resulting from re-emission efficiency and color rendering is not easy. Thus, since a typical white-light emitting device is implemented by combining the blue LED and the yellow phosphor, it lacks green and red components and thus it is difficult to express natural colors. Thus, the typical white-light emitting device is being limitatively applied to the screen of a portable phone or a notebook computer. Nevertheless, it has been widely used because it is easy to operate and remarkably cheap.
The light emitting device using the typical blue LED has used a method of increasing the content of a phosphor generating the second light of a red region in order to provide a high color rendering white light source.
FIG. 1 is a graph of wavelength vs. human being's eye response factor.
As shown in FIG. 1, a human being's eye response factor varies depending on the wavelength of light. When the eye response factor of light having a wavelength of approximately 555 nm is 1, the eye response factor of light having a wavelength of 520 nm is 0.71, the eye response factor of light having a wavelength of 580 nm is 0.87, the eye response factor of light having a wavelength of 605 nm is 0.57, and the eye response factor of light having a wavelength of 630 nm is 0.27.
That is, even though the same amount of light is generated, light having a wavelength of 555 nm and light having a wavelength of 630 nm have about a four-fold difference in efficiency.
FIG. 2 shows a light emitting spectrum of a white light reinforcing a red region in order to provide a high color rendering white light source.
Referring to FIG. 2, a correlation color temperature (CCT) and a color rendering index (CRI) are generally used as a performance index evaluating the characteristic of a white light, in which case the white light shows a dazzling, blue color as the CCT increases, and the white light is close to solar light (natural light) as the CRI increases. In particular, the CRI is used as an important index evaluating the performance of a white light. The CRI represents how the color of an object varies when the object is lit by solar light and by an artificial light source, in which case the color of the object when the object is lit by the solar light is defined as 100. That is, the CRI is an index representing how close is the color of the object under the artificial light source to when the solar light is lit and is represented by a figure between 0 to 100. The CRI of an incandescent lamp being currently distributed in the market is equal to or higher than about 80 and the CRI of a fluorescent lamp is equal to or higher than about 75.
As shown in FIG. 2, when a red region is reinforced in order to enhance the CRI of a typical light source 1 having CRI of about 80, it is possible to obtain a light emitting spectrum such as a typical light source 2 which has CRI of about 90.
However, since the eye response factor of light of a red wavelength band is too low there is a need to use many red phosphors that are expensive in comparison to a red or yellow phosphor, there is a limitation in that costs of phosphors remarkably increase, as mentioned above.